1. Field of the Invention
This invention relates to a method of detecting a liquid level and, more particularly, to a liquid level detection method which checks whether the level of a liquid contained in a tank is above a predetermined level by making use of an ultrasonic wave.
2. Description of the Prior Art
As a pertaining method well known in the art, there is one, in which an ultrasonic wave transmitter and an ultrasonic wave receiver are disposed side by side in a tank and above a predetermined liquid level such that their wave transmission and wave reception surfaces face the liquid surface. The wave transmitter transmits an ultrasonic pulse wave, and the wave receiver receives the ultrasonic wave that is reflected by the liquid surface, whereby the time required until the transmitted wave reaches the wave receiver is measured. With this method, however, if there occurs a wave on the liquid surface, the ultrasonic pulse wave from the transmitter is reflected randomly so that it is not accurately transmitted to the receiver. Therefore, this method cannot be applied to the detection of the level of liquid subject to strong vibrations such as the liquid level in an automotive fuel tank.
To improve this drawback, there has been proposed a method disclosed in Japanese Patent Application Sho58-210285. As shown in FIG. 5, in this method a wave transmitter T and a wave receiver R are disposed above and below a predetermined liquid level such their respective wave transmission and wave reception surfaces face each other. When both the transmitter T and receiver R are submerged in the liquid, an ultrasonic pulse wave from the transmitter T is transmitted through the liquid to be received by the receiver R. When the liquid level is below a predetermined level so that the receiver R is in air above the liquid level, the ultrasonic pulse wave from the transmitter T is reflected by the liquid surface because the acoustic impedance of the liquid is very low compared to the acoustic impedance of air. In this case, therefore, the transmitted ultrasonic pulse wave cannot be received by the receiver R, so that it is possible to effect checking as to whether the liquid level is above a predetermined level.
In this method, however, the transmitter T and receiver R have to be disposed in the tank such that the wave transmission and wave reception surfaces accurately face each other, and their distance from each other has to be set in a predetermined relation to the wavelength of the ultrasonic pulse wave, e.g. to be equal to an integral multiple of the half wavelength of the wave propagated through the liquid. Their positioning, therefore, is very cumbersome. Besides, in the case of the automotive fuel tank as noted above, the tank experiences shocks with the running of the automobile, so that it is liable that the wave transmission and reception surfaces are inclined or that the transmitter and receiver are deviated from the regular positions. In such a case, failure of reception of the wave would result. Further, when the liquid level is lowered below the receiver R, i.e., when it is not present between the transmitter T and receiver R, the wave is no longer received, and this would yield the same decision as that the liquid level is at the predetermined level.